A conventional drive device for a single-phase brushless motor has an H-bridge circuit and a diagnostic signal application circuit. The diagnostic signal application circuit applies one of motor terminals with a given voltage variation as a diagnostic signal in order to detect abnormality such as disconnection or short-circuit, which occurs inside a motor or a power supply path-thereof (for example, JP 8-47296A).
In this drive device, after all of switching elements of the H-bridge circuit are turned off, one of the motor terminals is supplied with the diagnostic signal from the diagnostic signal application circuit. In this situation, a reply signal that occurs in another motor terminal is detected, and the detected reply signal and the diagnostic signal are compared with each other, thereby detecting the abnormality of a motor side circuit.
According to the above proposed drive device, since the abnormality inside the motor or the power supply path can be automatically detected, an object to be driven which is connected to a motor shaft of the brushless motor can be safely operated.
However, in conducting abnormality diagnosis, because it is necessary to provide the application circuit that applies the diagnostic signal within the drive device of the brushless motor, the drive device cannot be downsized, and the costs increase.
Also, in the above proposed drive device, the abnormality diagnosis starts after all of the switching elements in the H-bridge circuit are turned off. However, the motor shaft may rotate by a force that is exerted on the motor shaft from the object to be driven or external vibrations by just turning off the switching element of the H-bridge circuit.
When the motor shaft rotates in this way, an electromotive force is developed in the motor. As a result, the above proposed drive device may not precisely conduct the abnormality diagnosis by the electromotive force.